Basic biology in many species pushes males and females toward different reproductive strategies, with males in some cases tending to seek as many partners as possible, and females -- because they invest so much more physically in producing offspring, and could even die in the process -- seeking one stable partner. The folklore in humans is that men tend to be more promiscuous than women, and its the biology that makes them do it.
In surveys, sure enough, this is what you find -- men report having, on average, more sexual partners than women. In one fairly recent study, sociologist Frederick Liljeros and colleagues found this pattern clearly among men and women in Sweden (they also found that the network of sexual contacts is highly skewed, with a few men and women having far more partners than most others). But this light article in the New York Times raises a fair question. Excluding homosexual contacts, the total number of times that females have sex has to be identical to the number of times males have sex; hence, the average number of sexual contacts for men and women has to be equal, simply by mathematics.
Liljeros et al. noted this in the paper I mentioned, and suggested that it might have something to do with males over-reporting their numbers, "bragging" about their impressive success. But I suspect there is probably also significant female under-reporting, especially as traditional norms in most societies take a dim view of female promiscuity. (One point the article doesn't explore is whether men and woman might interpret the word "sex" differently, leading to some encounters being viewed as sex by one partner, and non-sex by the other...but I'm guessing careful studies control for that by using some strict and unambiguous definition).
This systematic deviation of what people report from the underlying reality probably tells us something about mating strategies themselves; perhaps, for example, that females have a greater incentive than males to keep their promiscuity hidden. I came across an interesting essay on the topic, not by a biologist, oddly enough, but by Eric Raymond, a fellow linked in some deep way to the Open Source movement. No matter, it's a nice essay anyway, and informative. As Raymond argues:
Actual paternity/maternity-marker studies in urban populations done under guarantees that one's spouse and others won't see the results have found that the percentage of adulterous children born to married women with ready access to other men can be startlingly high, often in the 25% to 45% range. In most cases, the father has no idea and the mother, in the nature of things, was unsure before the assay.
These statistics cry out for explanation -- and it turns out women do have an evolutionary incentive to screw around. The light began to dawn during studies of chimpanzee populations. Female chimps who spurn low-status bachelor males from their own band are much more willing to have sex with low-status bachelor males from other bands.
That turned out to be the critical clue. There may be other incentives we don't understand, but it turns out that women genetically "want" both to keep an alpha male faithful and to capture maximum genetic variation in their offspring. Maximum genetic variation increases the chance that some offspring will survive the vicissitudes of rapidly-changing environmental stresses, of which a notably important one is co-evolving parasites and pathogens.
So maybe neither sex is the "more" promiscuous.